1. Go to dashboard and click on Products > Products with missing data:
2. Select to show 300 products per page, we suggest to don't select more than that.
3. Select all products on the page (use checkboxes) and from the bulk action select to download products to CSV or XLS.
4. In the downloaded file, delete all the columns which you are not going to modify, keeping only SKU column and those which you want to update. Make sure to do not change headers of columns.
5. After you fix the file, go to Tools and use Import/Export functionality to upload new data. Before any major update, please do backup by using Tools > Backup.